`delete(playlistWithID:)` can leave memory and disk disagreeing — contradicting its own doc

```swift
/// Removes a saved playlist — from memory and disk together, so the two never disagree
func delete(playlistWithID id: SavedPlaylist.ID) throws {
savedPlaylists.removeAll { id == $0.id } // memory: already gone
guard let playlistsStore else { return }
try playlistsStore.delete(documentNamed: id.uuidString) // disk: may throw
}
```

If the disk delete throws, the playlist is gone from memory but still on disk — and reappears on next launch. The call site in `LibraryView` catches and shows a toast saying "Playlist not deleted," while the row has already vanished from the list. Either delete from disk first, or restore the in-memory entry on failure.

Start at delete(playlistWithID:) and inspect the LibraryView call site that catches the error and shows the toast. Verify the failure path keeps memory and disk consistent, while a successful deletion removes the playlist from both; confirm the visible row and next-launch state match the result.

Written by the indexing model from the issue text.